1. Field of the Invention
The present invention relates to a laser diode, an optical disk device and an optical pick-up, and more specifically to, for example, a laser diode suitably used as a low-noise light source, and an optical device and an optical pick-up each of which uses the laser diode as a light source.
2. Description of the Related Art
Laser diodes are used as light sources of optical disk devices which read information from optical disks such as CDs (compact discs) and DVDs (digital versatile discs). In such a laser diode, light reflected by an optical disk to be returned to the laser diode, that is, so-called feedback light disturbs the oscillation state of the laser diode, thereby to cause noises. As a heretofore known technique of reducing feedback light-induced noises, a laser diode performing self-pulsation operation is effective. The principle of such a laser diode is to reduce the coherence of laser light to prevent disturbance of the laser diode caused by feedback light.
As a laser diode intended for self-pulsation operation, a two-electrode laser diode is known as described in, for example, Japanese Unexamined Patent Application Publication No. 2004-186678 and V. Z. Tronciu et al., Opt. Commun. 235 (2004) 409-414. FIGS. 9A and 9B illustrate an example of the configuration of a two-electrode laser diode in related art. In this case, FIG. 9A illustrates a plan view, and FIG. 9B illustrates a sectional view taken along line X-X of FIG. 9A.
As illustrated in FIGS. 9A and 9B, the two-electrode laser diode includes a laser stripe (a waveguide) 101 which extends throughout the length in a resonator length direction between a pair of parallel end surfaces 100a and 100b opposed to each other of a rectangular laser chip 100. The laser stripe 101 has a uniform width throughout its length. The laser chip 100 includes a semiconductor layer 103 forming a laser structure on an electrically conductive semiconductor substrate 102. The semiconductor layer 103 includes an active layer as well as an n-side cladding layer, a p-side cladding layer or the like (not illustrated). A section on a side close to the end surface 100a of the laser stripe 101 is a gain region 104, and a section on a side close to the end surface 100b is a saturable absorption region 105. The gain region 104 is formed so as to have a larger length than that of the saturable absorption region 105. Electrodes 106 and 107 are arranged on the gain region 104 and the saturable absorption region 105, respectively. A region between the electrodes 106 and 107 is a current non-injection region (an electrode separation region) 108. An electrode 109 is arranged on a back surface of the laser chip 100, that is, a back surface of the semiconductor substrate 102.
In the two-electrode laser diode with the above-described configuration, when a forward bias voltage is applied between the electrodes 106 and 109 arranged in upper and lower sections of the gain region 104, respectively, to inject a direct current, laser oscillation is performed. Moreover, when a reverse bias voltage is applied between the electrodes 107 and 109 arranged in upper and lower sections of the saturable absorption region 105, respectively, self-pulsation operation is performed.